The prior art of training and synchronizing a receiver that receives signals via multiple receiver antennas from a channel that introduces noise and intersymbol interference and which were transmitted by multiple antennas at the transmitter yields only a coarse channel estimate, and the training takes longer than is desirable due to an inefficient training sequence.
It has been recognized by the prior art that in orthogonal frequency division multiplexing (OFDM) systems that employ multiple antennas at the transmitter and multiple antennas at the receiver that training and synchronizing could be improved by using an orthogonal training sequence. To this end, a short orthogonal sequence known in the prior art was employed to perform the training and synchronizing. It is possible for OFDM systems to employ a short orthogonal sequence because given the long symbol duration employed in OFDM the channel, when described as a finite impulse response (FIR) filter, appears to have a length of 1, i.e., only a singly FIR filter coefficient is necessary to represent the channel. However, for high data rate CDMA and TDMA systems, which use a much shorter symbol duration, the channel appears considerably longer, e.g., for a 5 MHz bandwidth channel and 4 Msamples transmission rate the channel length is about 80 coefficients. Furthermore, in a MIMO system the required length of the training and synchronizing sequence is the product of the channel length and the number of transmit antennas. Thus, for a channel length of 80 and only 2 transmit antennas the required length of the training and synchronizing sequence is 160. Therefore, the prior art's known short orthogonal sequence cannot be employed for high data rate CDMA and TDMA systems.